1. Field of the Invention
This invention relates to heat modulation of a gas-fired heater, particularly a heater suitable for installation outdoors. This invention relates to digital heat modulation to incrementally modulate a heat input to a gas-fired heater by independently controlling and operating at least one solenoid valve to activate or deactivate a corresponding burner, such as an in-shot burner. The digital heat modulation method and apparatus of this invention can be easily adapted to receive one or more input data signals from a conventional single-stage or two-stage thermostat, so that a control algorithm of a modulator can provide an output signal to digitally control heat modulation.
2. Description of Related Art
Conventional outdoor or rooftop heating units are sized to a building heating design load. According to heating, ventilation and air-conditioning (HVAC) design practice, a heating unit preferably has a maximum capacity greater than the building heating design load. Generally, a rooftop heating unit is oversized 1.2 times to 1.7 times the building heating design load. An oversized heating unit responds quickly to a thermostat set point from a much lower set point condition, such as those associated with operation during evenings, weekends, and other unoccupied times.
A building heating design load includes an amount of heat needed to warm outside air that is mixed with return air, to ventilate the building. Increasing requirements and expectations for indoor air quality may require an HVAC system to introduce more outside air to a building. The amount of outside air introduced to a rooftop heating unit can range from about 20% to about 35% of the total air flow through the rooftop heating unit.
Many conventional rooftop heating units have a constant volume operation for controlling air flow to satisfy indoor air quality requirements. In a constant volume operation, a supply blower runs continuously in an on mode, regardless of whether the rooftop heating unit burners are firing.
As a result of the percentage of outside air introduced into the rooftop heating unit and constant volume operation, vent outlet air temperatures may drop quickly during off-cycle periods and may discomfort many occupants. To prevent these temperature fluctuations that may discomfort occupants, the heat input of conventional rooftop heating units is modulated.
In many conventional rooftop heating units, the heat input is adjusted by modulating a main gas valve. Thus, all burners of the rooftop heating unit are modulated simultaneously. This modulation approach limits turndown to about 3:1. With a turndown of about 3:1, excess combustion air is significantly increased and thus decreases the rooftop heating unit efficiency. To achieve a turndown of about 3:1 and to maintain efficiency these approaches require a multi-speed inducer fan to control excess combustion air. Further, if excess combustion air is controlled to maintain a constant air-to-fuel ratio, as the rooftop heating unit is turned down, the combustion products may condense in the heat exchanger or may condense in unintended portions of the heat exchanger. To avoid this condensation of combustion products and the subsequent corrosion damage to the heat exchanger requires a multi-speed indoor air blower to control condensation.
To provide some degree of heat modulation many conventional rooftop units use a two-stage main gas valve and are controlled by either a single-stage or two-stage thermostat. Conventional rooftop units equipped with a two-stage main gas valve can operate the burners at a full firing rate, at approximately 70% of the full firing rate and in an off condition, to maintain set points and to provide more continuous heat input to the rooftop heating unit while satisfying thermostat set points.
However, recognizing that for most operating hours of a unit the building load is less than 50% of the full firing rate, a rooftop heating unit with a two-stage main gas valve, which can only reduce the unit firing rate to about 70% of the full firing rate, will often provide heat input well above the heat load requirement. Therefore, to meet the heating load requirements, a rooftop heating unit will cycle between the on mode and the off mode, with the off-cycle periods increasing as the heating load decreases. As a result, many conventional rooftop heating units with a two-stage main gas valve do not improve the comfort level of the air circulated through the conditioned space of the building.
There is an apparent need for an outdoor or rooftop heating unit that reduces fluctuations in the supply air temperature to improve the comfort level of the air circulated through the conditioned building space.
It is also apparent that there is a need for a heat modulation method that incrementally modulates the heat input to a gas-fired heater for better control of the supply air temperature.
It is an object of this invention to provide a gas-fired heater having a heat modulation device that independently controls the activation of in-shot burners to modulate a heat input to a gas-fired heater over a wide range of overall firing rates.
It is another object of this invention to provide a heat modulation device that incrementally modulates a heat input to a gas-fired heater by independently operating solenoid valves to activate and deactivate corresponding in-shot burners.
It is another object of this invention to provide a heat modulation device that controls the activation or deactivation of a plurality of in-shot burners based only on feedback from a single-stage thermostat.
It is another object of this invention to provide a heat modulation device that manages the feedback from a single-stage thermostat, the initiation of the electronic ignition system of a gas-fired heater, the activation or deactivation of the main gas or combination gas valve of a gas-fired heater, and the activation or deactivation of independently operating solenoid valves.
It is another object of this invention to independently and/or sequentially control activation of a plurality of in-shot burners and to control a firing rate of at least one in-shot burner.
It is yet another object of this invention to control the amount of excess air in the gas-fired heater with a multi-speed inducer fan or with another flow restriction device.
The above and other objects of this invention are accomplished with a gas-fired heater, for example an outdoor or rooftop heater, having a plurality of burners, for example in-shot burners, each corresponding to a discrete section of a heat exchanger. The burners can have either approximately equal firing rates or different firing rates. In one embodiment of this invention, at least one burner has a variable firing rate.
Each burner is in fluidic communication with a fuel supply which furnishes a fuel to each burner. Within the burner the fuel is mixed with some portion of the air needed for complete combustion. Flames issue from the burners, mix with at least the remaining portion of air needed for complete combustion, and enter into the heat exchanger sections releasing heat and combustion products into the heat exchanger sections.
An induced draft fan, activated by a modulation controller, is preferably mounted to communicate with the combustion heat exchanger. The induced draft fan draws the combustion products through the heat exchanger and discharges the combustion products to the atmosphere.
A pressure switch mounted upstream of an induced draft fan or a centrifugal switch attached to the induced draft fan is responsive to a pressure or a rotational speed, respectively, within a range of normal operation. A pressure or rotational speed within a range of normal operation causes a pressure switch or centrifugal switch to electrically energize an electronic ignition system.
Once energized, an electronic ignition system electrically communicates with an ignition source or sources near one or more of the burners or near a pilot burner, the main gas valve or combination gas valve including a pilot valve section and a flame sensing device. An electronic ignition system safely and reliably lights the burners and any pilot burner.
The gas-fired heater has a supply blower which draws air from both the conditioned space of the building and the outside air. The blower moves the air over the heat exchanger. The heat exchanger transfers heat by convection and/or conduction to the air. The heated air is forced through a conduit, a duct system for example, and circulated throughout the conditioned space of a building.
At least one valve, such as a solenoid valve is positioned with respect to a corresponding burner. Each valve is independently controlled and/or moved between an open position and a closed position, to control fuel flow from the fuel supply to the corresponding burner.
A modulator electrically communicates with each valve and emits a signal that is used to control movement, if any, of each valve, such as between an open position and a closed position. The modulator of this invention incrementally modulates the heat input rate to the gas-fired heater by independently moving at least one valve to the open position or the closed position.
A single-stage or two-stage thermostat, preferably a single-stage thermostat, electrically communicates with the modulator to provide feedback on the heat input rate by closing the thermostat circuit to signal that the heating load is not met or by opening the thermostat circuit to signal that the heating load is met.
In a method for modulating the heat input to the gas-fired heater, the modulator emits a control signal, preferably but not necessarily a dedicated signal, to each solenoid valve to independently operate or control each solenoid valve, such as between the open position and the closed position. With the solenoid valve in the open position, the fuel flows from the fuel supply to the corresponding burner. The modulator can also activate any burner by emitting a control signal to ignite and combust or burn the fuel. Additional solenoid valves can be independently or collectively operated or controlled to move from the closed position, which prevents or restricts fluidic communication between the fuel supply and the corresponding burner, to an open position allowing fluidic communication between the fuel supply and the corresponding burner. The dedicated signal selectively activates the corresponding burner. Thus, the heat input to the gas-fired heater can be incrementally modulated.
The modulator of this invention uses a control algorithm that can receive a signal emitted from a conventional single-stage or two-stage thermostat and in response emit one or more control signals to one or more of the burners and to an electronic ignition system, to digitally control modulation.